The present invention relates to a signal selector system for selecting a signal for control of various parameters in a process facility. More particularly, the present invention pertains to automatic control systems in power plants and impooving reliability of signals which are generated as representative of various critical operating parameters and which serve as inputs to various control systems, for example, the feedwater control system or the pressure control system.
The signals which are used to control the various systems in, e.g., a nuclear power plant are generated by sensors (for example thermocouples, pressure sensors, etc.). The sensors continuously monitor the critical operating parameters of the nuclear power plant.
For example, if the signal from a pressure sensor fails and produces a "low" signal, i.e., a signal which is not indicative of the actual pressure, the control system receiving this signal would automatically take action to increase the pressure. Such an increase would not be appropriate to the actual conditions. Alternatively, the "low" signal could result in a false alarm causing a plant shut down. The shut down of a power plant for even a short period of time can cause an enormous economic loss to the plant and possible disruption of service to the public. In addition, it may be difficult to locate the source of the problem, i.e., the failed sensor, in the absence of additional indication to an operator of the source of the failure. A search for the problem area would likely cause additional, costly time delays. Regardless of the parameter under control, it is apparent that the reliability of the signals representing the measured quantity is of critical importance if the system is to be properly controlled.
One conventional method of improving control signal reliability is to use multiple redundant sensing devices, e.g., two or three sensors sensing the same parameter. An operator chooses between the redundant measurements via a switch on a console. Normally the operator selects one of the signals (called the preferred signal) for control. A back-up signal is generally used only if a problem or erroneous signal at the preferred signal is detected. However, this method requires constant supervision by the operator and the system is only as reliable as the operator. Other systems have been proposed where a back-up signal is automatically selected if the preferred signal falls outside a predetermined range. However, no indication that the back-up signal has been selected is generally provided to the operator.
Other types of signal detectors for other applications using automatic signal selection have also been proposed. For example, in U.S. Pat. No. 4,414,540 issued to Dickenson, an automatic signal selector is disclosed for selection of a valid signal representation of steam pressure for utilization by a turbine control system wherein steam pressure is a controlled parameter. Each pressure signal produced is applied to a corresponding range detection circuit which generates an alarm signal whenever the pressure signal is either higher or lower than corresponding pre-selected high and low point values representing maximum and minimum acceptable values of steam pressure. Based on the alarms, a signal within the acceptable range is automatically selected.
Also, U.S. Pat. No. 3,588,857 issued to Gessner discloses an automatic fail safe monitor wherein a signal corresponding to the difference between a pair of redundant signals is compared to a reference signal. If the difference exceeds a predetermined threshold, an alarm device is activated. The comparator has independent dual outputs so that in the event of a failure of one output, the other outputs function normally.
U.S. Pat. No. 3,431,557 issued to Thomas et al. and U.S. Pat. No. 4,200,864 issued to Gillet et al. disclose similar systems using redundant signals whereby the difference in the signal is compared to a threshold for attempting to determine a signal failure.
However, these patents do not disclose systems that are fully adequate to meet the needs of certain applications, for example, control systems in nuclear power plants. These prior systems are not fully suitable for all uses as they do not provide sufficient data to the operator to determine the operational state of the systems. For example, in systems that have only manual control by an operator, there is inadequate reliability because the system requires constant supervision. Also in previous systems, the operator has no clear indication of a signal problem and could switch to a bad channel. Likewise, in solely manual signal selecting systems, several rapidly developing transients will result if the preferred signal fails and the second back-up signal is not promptly selected. The plant may be shut-down before the operator has time to react.
The automatic signal selecting systems discussed above provide more reliability, but do not provide adequate means for automatically switching at an appropriate time and for giving an operator sufficient data to make an informed decision concerning the system being controlled. None of the previous systems discloses an automatic signal selector which compares the signals from two sensors to one another and to a first pre-selected threshold value and by either comparing the preferred signal to a pre-selected second threshold value or by performing an absolute comparison of the two signals to one another. The prior systems do not utilize a two-step comparison to determine whether the first preferred signal is acceptable, i.e., an accurate representation of the value of the controlled parameter. Nor do the automatic signal selector systems disclose an arrangement for allowing an operator to decide that the chosen signal is inappropriate and to switch to a back-up signal based on information he receives at the console.